In such a device, heat energy from the source is transmitted to the leakproof connection via the corresponding end of the glass tube. This connection has a sealing gasket which is thus raised to a high temperature while the gaseous compounds are being deposited or while the tube is being collapsed. Unfortunately, it is very important for the gasket to maintain a good seal so as to prevent any risk of the gaseous compounds being exhausted into the medium surrounding the apparatus, and to avoid any contamination of the deposit by the surrounding medium.
A known solution to the problem of the leakproof connection between the gas feed means and the end of the tube being overheated consists in designing a gasket of a complex shape and in making it out of special materials so as to guarantee that the connection remains sealed at the temperatures generated by a blowtorch. Unfortunately, that solution increases the cost of manufacturing a preform.
Another known solution to the above problem consists in holding the glass tube on the lathe by means of a tubular endpiece. In this way, the end of the tube is located at a certain distance from the leakproof connection and the corresponding chuck which come into contact only with the tubular endpiece. The inside diameter of the endpiece is slightly greater than the outside diameter of the glass tube and it is assembled thereto by adhesive acting on an overlap zone where the glass tube is received in the tubular endpiece. This establishes a discontinuity which enables a large part of the heat energy to be dissipated by means of the end section of the glass tube. Nevertheless, that solution also increases cost due to making and assembling the endpiece.
After a preform has been subjected to an internal deposition method, similar problems arise when the preform is collapsed. At that time, the preform is held at its ends by the chucks. While the preform is being heated by the heater means for the purpose of collapsing it, the thermal radiation generated by the heater means heats the chucks that are holding the preform, thereby running a risk of the chucks becoming jammed.
Finally, similar problems arise when drawing an optical fiber from a preform. At that time, the preform is held vertically at one of its ends by means of a chuck. While the preform is being heated by the fiber-drawing oven, the thermal radiation it generates heats the chuck that is holding the preform and that also can run the risk of the chuck becoming jammed.